A magnetoresistive magnetic head is used as a read sensor in a high recording density magnetic recording technology including a hard disk drive as a main constituent thereof, and influences the performance of the magnetic recording technology. In an environment in which the recording densities of magnetic recording and reproducing devices have rapidly increased, it has not been achieved to realize a magnetic recording and reproducing device having a sufficiently high recording density, in particular to realize a magnetoresistive magnetic head acting with sufficient sensitivity and outputs to the external magnetic field in the read section of the magnetic recording and reproducing device, and further to obtain a preferable characteristic with sufficiently good stability in the prior art, and it has been difficult to realize the function as a storage device.
In recent years, it has been known that the magnetoresistance effect of a multilayer film formed by stacking ferromagnetic metal layers via a nonmagnetic metal layer, so-called giant magnetoresistance effect, is strong. In the past, in the case in which the magnetoresistive film is used as a magnetic sensor, a so-called CIP-GMR film, which flows an electric current in-plane of the layered film, has been used. Further, a so-called CPP-GMR film, which is a perpendicular current type giant magnetoresistive film flowing an electric current in the film thickness direction of the layered film, has also been under investigation. A TMR film, which uses tunneling magnetoresistance effect flowing an electric current in the film thickness direction of the layered film in a similar manner, has also been under research and development.
A disadvantage of the magnetic head using the conventional magnetoresistive magnetic sensor, is that high resolution in the linear density direction is not achievable. In the conventional magnetic head, the resolution in the linear density direction is determined by a read gap defined by the distance between a pair of magnetic shields. Although it is conceivable that the high resolution with respect to the linear density direction can be achieved by narrowing the distance between the magnetic shields, since the magnetoresistive magnetic sensor needs to be disposed in the gap of the magnetic shields, it is obvious that the gap of the magnetic shields is geometrically limited by the thickness of the magnetoresistive magnetic sensor.
In more detail, it is desirable that the part of the magnetoresistive sensor for detecting the magnetic field, the part thereof called a soft magnetic free layer here, is disposed in the vicinity of the center of the magnetic shield gap. This is because, if the soft magnetic free layer described above is disposed so as to have contact with one of the magnetic shields or to be extremely close thereto, a larger amount of leakage magnetic flux from the recording medium to be detected flows into the thicker magnetic shield with high magnetic permeability than the amount of the leakage magnetic flux flowing into the soft magnetic free layer of the magnetoresistive magnetic sensor for detecting the leakage magnetic flux, which lowers the output of the magnetoresistive magnetic sensor.
Here, the thickness of the magnetoresistive magnetic sensor will be described. Although there are cited the CIP-GMR film, the TMR film described above, and further a CPP-GMR film, which is a magnetic sensor under research and development, as the magnetoresistive magnetic sensor presently used for the magnetic head for the hard disk drive, the configuration widely used as a fundamental configuration is a synthetic ferri-magnet-type spin valve structure. Specifically, it has a fundamental configuration composed of a base film, an antiferromagnetic film, a first ferromagnetic pinned layer, an antiparallel coupling layer, a second ferromagnetic pinned layer, a non-magnetic intermediate layer, a soft magnetic free layer, and a protective layer. The thickest of all of these constituents is the antiferromagnetic film, and the thickness thereof is conceivably about 8 nm. In estimating the total thickness by citing typical thicknesses, it makes 4+8+2+1+2+2+4+2=25 nm from the base film side, and consequently, the total film thickness of the configuration described above becomes approximately 25 nm. Since the soft magnetic free layer among these constituents is disposed on the end of one side in the sum total, in order for disposing the soft magnetic free layer so as not to be too close to the magnetic shield described above, it is necessary to add an electrically conductive film of about 5 nm, for example, to the protective film.
Therefore, in the magnetic head using the magnetoresistive magnetic sensor of the conventional type, the magnetic shield gap has a limitation of about 25+5=30 nm because of the thickness of the magnetoresistive magnetic sensor, and even though the gap can be reduced to some extent by an effort of reducing the thicknesses of the various films, it is quite difficult to manufacture the magnetic shield gap to be narrower than 30 nm, in particular equal to or narrower than 20 nm.
Meanwhile, as a technology, which has been proposed before, there is cited a differential magnetic head. Although the differential magnetic heads in the broad sense include a magnetic head for detecting the magnetic flux flowing in the magnetic gap using a magnetic yoke having the magnetic gap, since it is difficult to realize the magnetic yoke type differential magnetic head in the microscopic magnetic head having a track width of equal to or smaller than 0.1 μm in recent years, the magnetic yoke type differential magnetic head is to be excluded from the extent of the discussions. What is described here as the conventional technology is a magnetoresistive magnetic sensor for reading a signal at a position distant in the linear density direction, the magnetic head for performing a differential operation with respect to the signal at the position distant in a manner described above. In Japanese Patent Publication No. 7-85426 (“Patent Document 1”), there is a description of a dual-element type magnetoresistive head using an anisotropic magnetoresistive sensor. As what is similar to the magnetic head technology in recent years, as described in Japanese Patent Publication No. 2003-69109 (“Patent Document 2”) and Japanese Patent Publication No. 2004-227749 (“Patent Document 3”), it can be manufactured so that the two magnetoresistive sensors disposed at positions distant in the linear density direction are formed, and the signals thereof cancel each other with respect to in-phase magnetic fields, and reinforce each other with respect to hetero-phase, namely differential magnetic fields. What these technologies seek for is to determine the resolution in the linear density direction, which has ever been determined by the magnetic shield gap, by the distance between the two sensors by performing differential operation of the signals of the two sensors disposed at the positions distant in the linear density direction, and in other words, the object thereof is to realize the higher resolution than in the conventional magnetic head.
The problem of the dual-element type differential magnetic head described above is difficulty in mass production as a commercial product. This is because, due to the nature of performing the differential-operation of the two sensors to obtain one signal, the defective and variation fraction, which is double as high as that of the conventional magnetic head formed of a single sensor, should be caused. For example, as the variation of the magnetic beads caused in the manufacturing process, the output variation and the variation in waveform symmetry are cited. It is assumed that the conventional magnetic head can be manufactured with the output variation suppressed to, for example, ±10%. If the dual-element type differential magnetic head is manufactured with the same technology, the output variation becomes ±20%, which is a simple addition of the variations of the two elements. Further, it is assumed that the screening of the magnetic head is performed to screen only the heads with preferable waveform symmetry in order for suppressing the variation in waveform symmetry to ±20%. If it can be performed with the defective fraction of 5% in the conventional magnetic head, the defective fraction of the dual-element type, which requires to screen those with both of the two elements having preferable waveform symmetry, is doubled to be approximately 10%. As described above, although the high resolution must be obtained in the dual-element type differential magnetic head, since the problem in the manufacturing process is serious, it has never been realized to the market where the high recording density and high reliability have been required in recent years.